How can I apply the same change to multiple channels in Graph Editor?
Use the eye icon to show only the curves you want to edit, then select all keyframes you want to edit, then press G to move, G X to move along time, G Y to move along values, S X to scale along time, S Y to scale along values. Confirm with enter or Left click.

You can set the pivot point of scaling with the usual pivot icon (Bounding box, individual center or 2D cursor).